1. Field of the Invention
The present invention relates to a polyester composition containing, in a substantially homodisperse distribution, finely divided crosslinked polymer particles which have been prepared in an emulsion-polymerization process and exhibit a narrow grain-size distribution. The polymer particles are incorporated into the polyester during the synthesis thereof.
The invention also relates to molded articles comprising the above-mentioned polyester composition and preferably being in the form of a stretched film, particularly a biaxially stretched film, which may even form part of a multilayer film.
The invention furthermore relates to fibers or filaments formed from the polyester composition according to the invention or comprising an addition of this composition to other polymers.
Moreover, the invention pertains to the use of the film, particularly as a support material for magnetic recording elements or as a capacitor film, and also to the use of the fibers or filaments, preferably in the production of tire cord.
Due to their outstanding properties, polyesters, in particular polyethylene terephthalate or polyester copolymers, which are especially in the form of oriented films, preferably biaxially oriented films, are used in many technical fields, for example, as a dielectric in capacitors, as a support material for video, audio and computer tapes, as stamping foils and the like.
For the various fields of application, polyester films should possess a number of specific properties, and these properties must either be systematically adjusted in the production process or must be pre-formed already in the raw material. Above all, a systematic adjustment of surface properties, particularly of the roughness structure of the surface and of the slip and abrasion behaviors which are connected therewith, must be taken into account, in view of the high processing speeds which are presently employed and in view of the permanent stresses occurring in use.
If the films are intended for use, for example, as supports for magnetic recording elements in audio, video and computer techniques, they must have a uniform and good slip behavior and a high abrasion resistance and, in particular, they must meet especially high requirements in respect of uniformity of surface structure. In particular, relatively large particles should not be present on the film surface, since these will form undesirable elevations, for example, upon coating with a magnetizable layer. When the film is, for example, used as an information carrier, these elevations result in a loss of information and thus limit or affect the usability of the film. Similarly, indesirable elevations produced by coarse particles give rise to difficulties in the metallization of capacitor films. For this purpose, good surface properties are required, which are diminished to a considerable extent by nonuniformity of the pigment.
High requirements regarding uniformity of surface must also be met, when polyester films are used as stamping foils. In other technical applications good transparency of the films is required in the first place.
2. Description of the Prior Art
It has been disclosed in the prior art to overcome or at least alleviate the disadvantages by adding pigments of a very broad grain-size distribution to polymers, in such a way that, for example, the inorganic particles are classified or catalyst precipitation is influenced by the choice of catalysts and by the choice of catalysts and by the choice of the type and quantity of the phosphorus stabilizers which are added in the polyester production.
A detailed summary of the prior art and of the methods employed to overcome the disadvantages in the structuring of film surfaces, by catalyst precipitation or by the addition or inorganic particles, is given in U.S. Pat. No. 4,320,207. U.S. Pat. No. 4,320,207 has proposed to incorporate crosslinked particles into polyester films, in order to improve, in particular, the affinity between the added particles and the polymer matrix, the crosslinked particles being formed by pulverizing a crosslinked polymer having a specific surface area of at least 1 m.sup.2 /g and a pore volume of at least 0.1 ml/g. The described improvements which are achievable with these particles, due to an improved affinity for the polyester--either by filling pre-formed cavities with matrix polymer or by true covalent bond of the particles with the polymer matrix--require great technical expenditure in the preparation of the particles. As disclosed, a porous crosslinked copolymer must be polymerized in an emulsion-polymerization process with the addition of linear polymers and solvents to produce the polymer particles which are described as additives for film-forming polyesters.
Moreover, the organic solvent added and the straight-chain polymeric compound must be extracted to obtain a good grindability and to prevent disturbing foams. It is also necessary to pre-grind the polymer to a grain size of approximately 10 .mu.m in a jet mill. The desired grain-size distribution must, in addition, be adjusted by at least one grinding operation in a pearl mill.
It is still a fundamental disadvantage of this prior art process that--in addition to the great techncial expenditure--the kind of adjusting the grain size of the particles inevitably results in a particular grain-size distribution, involving the problem of oversize grain, i.e. particles which have sizes exceeding the average size.
It is therefore an object of the present invention to provide a polyester raw material containing, in a substantially homodisperse distribution, finely divided crosslinked polymer particles which show a narrow grain-size distribution in the polyester material and possess a good affinity for the raw material; which result in a good distribution upon incorporation into the raw material; and which give a structured surface without any undesirable excessive structures (in the positive and/or negative region), agglomerates or coarse surface elevations, when they are used for the production of molded articles, particularly films which are preferably in a stretched form.